Recently, it has been discussed that the primordial black hole (PBH) observation can be used as a probe of strongly first-order EWPT [1]}, which is motivated by the work by Liu et. al. [2]} where PBH production and its abundances in general first-order phase transition have been studied.
It has been known that large density contrast in the early Universe can gravitationally collapse into the PBHs [3]}, [4]}, [5]}.
Sufficiently large density contrast can be generated by the first-order phase transition [6]}, [7]}, which can produce PBHs.
The mass of PBHs produced by the first-order EWPT is about \(10^{-5}\)  of the solar mass, which can be observed by microlensing observations such as Subaru Hyper Suprime Cam (HSC) [8]}, Optical Gravitational Lensing Experiment (OGLE) [9]}, PRime-focus Infrared Microlensing Experiment (PRIME) [10]} and Nancy Grace Roman (Roman) Space Telescope [11]}.
Therefore, exploration of such PBHs at these observations can be used to probe models of strongly first-order EWPT.
